Optimize loader

Description

Improve performance by cythonizing part of the loader

Changes proposed in this merge request:

• Move _loader.loadelement to cython
• Move _loader.loader.sort_dependencies to cython

Benchmarks

Run on debian-stack, run 7 times, dropped the first two.

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commit	action	python_version	max_memory	time
bschubert/optimize-loader - 2d56d97a	show	py37	1660	60.06
master - 9656b2cb	show	py37	1664	62.14

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